The cat family has served as an excellent model system for the study of viral-induced cancers. The Felidae has been divided into as few as 3 to as many as 20 different genera. To make any meaningful assumptions about the transmission of genes (viral or non-viral) across species lines within this family, a better understanding of its phylogeny is necessary. The aim of this study is to make in-depth comparative studies at the chromosomal, protein and DNA levels among members of the cat family. Mitochondrial DNA (mtDNA) has been chosen as an evolutionary probe. The molecule evolves rapidly, is maternally inherited and provides a high degree of resolution among closely related species. mtDNA has been isolated from the domestic cat, lion, cheetah and clouded leopard. The mtDNAs have been digested with 17 different restriction enzymes and cleavage maps have been constructed. The intra- and interspecific variations of these 4 index cat species have been measured. The construction of the cleavage maps have also aided in the preparation of mtDNA clones. The mtDNA clones will be used as molecular probes of mtDNA from each Felidae species (35 of 41) using "Southern" analysis of high molecular weight DNA from cultured cell lines. The comparative restriction maps of the mtDNA from the various species will be used to construct a molecular phylogeny of the Felidae. This phylogeny will be compared to phylogenetic topologies of the Felidae derived by other molecular and morphometric measurements.